An extruded member has been widely used as for example, a raw tube for a photoconductive drum substrate, a component for an office automation equipment, an architectural material, and an exterior member. In recent years, the demand for quality of a surface texture of an extruded member is getting stricter. Among other things, the demand for quality of appearance and surface roughness of an extruded member is especially getting stricter.
As appearance defects and surface roughness defects of an extruded member caused during extrusion molding, air entrapment defects can be exemplified. Such air entrapment defects not only cause appearance defects and surface roughness defects but also cause deterioration of an exterior appearance of a plastically formed product or an increased surface roughness when plastic forming, such as a drawing process or a bending process, is executed at the following step.
Furthermore, when a drawing process is executed at the following step, the air entrapment defects will be drawn in the drawing direction, which in turn causes enlarged ranges of the appearance defects and surface roughness defects. Especially, in the case of using a drawn member obtained through a drawing process as a raw tube for a photoconductive drum substrate, the surface roughness will be drawn in the axial direction of the raw tube, causing problems that elongated line-shaped defects will be appeared on a printed image.
As the causes for generation of air entrapment defects, a poor cut-surface of a discard can be exemplified. Considering it, a configuration of an extrusion die has been improved. For example, according to Japanese Unexamined Laid-open Patent Application Publication No. 2002-1422 (hereinafter referred to as “Patent Document 1”), it discloses forming of a concave portion in the center of the upstream end surface of the extrusion die (more specifically, porthole die) as the improvement of the configuration of the extrusion die. The discard is also called an extrusion remain which is apart of a billet remained in a container after extrusion.